In the past, a prepreg has been formed by impregnating a fabric substrate with a resin composition containing a thermosetting resin and drying thus-obtained substrate by application of heat until the resin composition is in a semi-cured state (see, Patent Literatures 1 to 3, for example). Besides, a metal-clad laminate is formed by disposing metal foil on the prepreg formed in the aforementioned manner, and a printed wiring board is formed by patterning the metal-clad laminate to make a patterned conductor. Thereafter, a package (PKG) is prepared by mounting a semiconductor element on the printed wiring board and enclosing the element. To impart excellent quality to such a package, a multilayer substrate with high reliability such as connection reliability in which warpage and the like is suppressed has been required.
One approach to improve connection reliability of a multilayer substrate is to reduce a coefficient of thermal expansion (CTE) of a substrate material. When a difference in CTE between the resin composition in the multilayer substrate and patterned Cu or plating in a through-hole is decreased, stress caused by thermal expansion or thermal contraction in a connection reliability test (e.g., thermal shock test) can be reduced. Accordingly, the connection reliability can be improved. Specifically, the approach may be to add an inorganic filler into the resin composition with which the substrate is to be impregnated.
Another approach to improve the connection reliability is to reduce elasticity of the substrate material. In the approach, it is planned that the connection reliability is improved through a principle in which a low elastic material (resin) buffers the stress caused by thermal expansion or thermal contraction in the connection reliability test (e.g., thermal shock test). Specifically, this approach may be to appropriately select resins to be contained in the resin composition with which the substrate is to be impregnated to make the resin composition lower in elasticity, or to reduce an amount of the inorganic filler added into the resin composition.